1. Field of the Invention
The present invention relates to a valve unit of an internal combustion engine in which a valve characteristic of an intake valve or an exhaust valve is continuously controlled.
2. Description of the Related Art
In a valve unit of a multicylinder reciprocating engine (internal combustion engine) mounted on an automobile, in order to reduce fuel consumption by exhaust gas measures or by improving pumping loss, a variable valve in which a characteristic of an intake valve (or an exhaust valve) is continuously and variably controlled is incorporated in a head part of a cylinder head covered with a rocker cover.
In most variable valve units, a structure is used in which a characteristic of an intake valve, e.g., an opening/closing timing or a valve lift amount is continuously varied by a rotational displacement of a control shaft received from a cam. A variable valve unit of this type is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-299536.
In most methods of installing such a variable valve unit, a method is used in which, a cylinder head is mounted on a cylinder block on a main line for assembling engines, and each part of the variable valve unit is attached to each corresponding section of the cylinder head, thereby assembling the entire variable valve unit.
In recent times, in order to increase production efficiency of the main line, on the main line, work in which only camshafts and valves are attached to a cylinder head is performed. On a sub-line separate from the main line, a method is used in which a variable valve unit constituting a part of a cylinder head from a camshaft to a valve is modularized.
That is, only the variable valve unit, which is troublesome in assembly, is modularized on the sub-line, the modularized variable valve unit is returned to the main line, and the variable valve unit is attached to a cylinder head (which is already equipped with camshafts and valves). By doing so, a measure is used in which a working process taking much time is reduced on the main line. Assembling methods of such a type are disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-299536 and Jpn. Pat. Appln. KOKAI Publication No. 2005-299538.
Incidentally, the variable valve unit is required to continuously control valves of cylinders in accordance with the same valve characteristic so that a set performance can be exhibited in any operational state of an engine. For that purpose, the variable valve unit is required to undergo adjustment work for adjusting a valve drive output in accordance with a cam profile of each cam for each cylinder, thereby eliminating variation between cylinders.
However, in the above adjustment for eliminating variation between cylinders, troublesome and considerably time-consuming fine adjustment work for making relationships between cams and parts of the variable valve unit for receiving the cams with respect to the respective cylinders so that the continuously variable valve characteristic can be appropriately exhibited is required.
Particularly, in the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-299536, a contrivance is employed in which an adjustment mechanism is incorporated in the variable valve unit, the adjustment mechanism having a structure in which a position of a part for receiving a cam is made adjustable, thereby facilitating the adjustment work. For this reason, the adjustment work can be performed only after the variable valve unit provided with parts for receiving cams of a camshaft is attached to the cylinder head provided with camshafts. Therefore, on the main line for assembling engines, considerably time-consuming adjustment work (adjustment for eliminating variation between cylinders) is still required, which is a factor for causing stagnation of the main line.
Furthermore, in the adjustment for eliminating variation between cylinders, not only simply making positional relationships between cams and parts for receiving the cams uniform, but also making uniform the valve characteristics on the basis of the continuously variable control shaft is needed. Accordingly, work for attaching a sensor for detecting a rotational displacement of the control shaft, and work for adjusting the sensor is required on the main line. Such work is also a factor causing stagnation of the main line. Particularly, the sensor is an important part for continuously and variably controlling the valve characteristic. Therefore, the attaching of the sensor must be performed in consideration of maintenance because maintenance of the sensor is required in a state where the assembly of the engine is finished or after the engine is completed as a product. Considering these requirements, considerably difficult problems must be solved to eliminate the stagnation of the main line.